1. Field of the Invention
The present invention relates to an electroluminescent device, and more particularly, it relates to an improvement in a luminous layer which emits light by application of an electric field.
2. Description of the Prior Art
An organic electroluminescent device, which is employed for displaying characters, graphics etc. or as a light source for a liquid crystal display, is generally formed by a back electrode of aluminum foil or the like, a reflecting insulation layer of an insulating material prepared by an organic dielectric substance mixed with powder of barium titanate etc., a luminous layer of an organic dielectric substance in which electroluminescent particles, or fluorescent particles, are dispersed, a transparent electrode and a transparent resin casing film. In such an electroluminescent device, AC voltage is applied across the back electrode and the transparent electrode to provide an AC electric field to the fluorescent particles contained in the luminous layer, thereby to excite the same for light emission.
It has been pointed out that luminance and life of such an electroluminescent device are extremely influenced by external conditions such as the value and the frequency of the AC voltage applied across the electrodes, as well as by internal conditions of the luminous layer, in particular, such as absorption of external moisture.
As the prior art for preventing reduction in luminance of an electroluminescent device and providing longer life, Japanese Patent Publication Gazette No. 28377/1965, for example, discloses an electroluminescent device which comprises a luminous layer containing fluorescent particles having surfaces covered with microcrystals of titanium oxide or barium titanate. Such titanium oxide or barium titanate incidentally serves as a moisture-proof material for protecting the fluorescent particles against external moisture. In this prior art example, the process of adhering the microcrystals of titanium oxide or the like to the surfaces of the fluorescent particles includes steps of mixing the fluorescent particles into a suspension of titanium oxide or the like and performing heat treatment under a high temperature (550 -- 610.degree. C.) thereby to manufacture a block containing the fluorescent particles, and thereafter pulverizing the same.
However, the fluorescent particles are deteriorated by the high temperature in the heat treatment step, while the surfaces thereof are partially uncovered and exposed by pulverization in the pulverizing step. Due to such problems, the fluorescent particles are ineffectively protected against moisture in practice, and hence the luminous layer cannot be satisfactorily prevented from reduction in luminance while its life cannot be sufficiently increased.
In order to solve such problems, Japanese Patent Laying-Open Gazette No. 264095/1985, for example, provides an electroluminescent device, in which the surfaces of fluorescent particles are covered with a moisture absorbent in place of the aforementioned moisture-proof material. As described in this gazette, external moisture is absorbed by the moisture absorbent to be prevented from penetration into the fluorescent particles for a considerably long time. Thus, the device is protected against the moisture for a long time, whereby the luminous layer can be prevented from reduction in luminance and its life can be increased.
In this prior art example, however, although the device can be protected against moisture until the amount of external moisture absorbed by the moisture absorbent reaches the maximum absorption volume, i.e., a so-called saturation level, the absorbed moisture is bled off toward the fluorescent particles after the amount of absorption reaches the saturation level to rapidly deteriorate the fluorescent particles. Thus, the life of the luminous layer cannot be basically increased by this prior art example.